1. Field of the Invention
The present invention relates to a magnetic memory device, e.g., a read-system circuit of a magnetic memory device.
2. Description of the Related Art
A magnetoresistive element is known as a nonvolatile memory element. The magnetoresistive element includes a free layer and fixed layer made of a magnetic material and a nonmagnetic layer sandwiched between them, and takes different resistance states in accordance with the direction of magnetization in the free layer. A magnetic random access memory (MRAM) is a magnetic memory device that stores data by using this resistance state difference.
As a data write method, a so-called spin injection write method has been proposed (U.S. Pat. No. 5,695,864) in addition to the conventional, so-called magnetic field write method using a current magnetic field.
In spin injection write, the current of electrons spin-polarized by the magnetic moment in the fixed layer of the magnetoresistive element is supplied to its free layer. Since the direction of magnetization in the free layer changes in accordance with the direction of this electron current, specific data is written in the magnetoresistive element. Unlike the magnetic field write method, this write method can exert a direct action on the magnetoresistive element. Accordingly, no write error occurs in adjacent memory cells. Another advantage is that the amount of electric current required for write reduces as the cell size decreases.
Data read is performed by supplying a read current to the magnetoresistive element, converting the resistance value into a current value or voltage value, and comparing this current value or voltage value with a reference value.
The magnetoresistive element uses the same data holding mechanism for the magnetic field write method and spin injection write method. Therefore, a partial peripheral circuit configuration can be shared by MRAMs using the two write methods. However, the two write methods require some different peripheral circuits because the write principles are different.
One peripheral circuit required to have an optimum configuration in accordance with the write method is a read-system circuit. The two write methods use the same principle for data read. Since the write principles are different, however, a read-system circuit used together with the magnetic field write method cannot be simply used together with the spin injection write method. Accordingly, demands have arisen for implementing a read-system circuit suited to the spin injection method.